To make a large-capacity optical transceiver have a smaller size and lower power consumption, a technology to monolithically integrate an optical device on a substrate having a silicon waveguide plays an important role. As a configuration that makes the modulation efficiency higher for a modulator on a silicon substrate, and reduces the power consumption and the device size, ring assist (RA) modulators have been proposed (see Non-Patent Document 1, for example). An RA modulator has multiple ring resonators arrayed in a phase shifter to make the effective interaction length greater.
FIGS. 1A-1C are schematic views of a conventional optical semiconductor device 1000. The optical semiconductor device 1000 has a laser oscillator 1010 and a ring-assist-type Mach-Zehnder (MZ) modulator 1020 integrated on a not-illustrated silicon substrate. The laser oscillator 1010 includes two mirrors 1011 and 1012 facing each other and a ring resonator 1013. By inserting the ring resonator 1013, a filter characteristic is obtained that transmits wavelengths only in the neighborhood of the ring resonance wavelength as illustrated in FIG. 1C to execute laser oscillation that centers around the ring resonance wavelength. By making the shape and size of the ring resonator 1013 be equivalent to the shape and size of the ring resonator 1023 of the ring-assist-type MZ modulator 1020, the laser oscillation can be executed at a wavelength with which the highest modulation efficiency of the modulator 1020 is obtained.
The ring resonance wavelength may vary due to manufacturing variations even if the size of the ring resonator is designed to have a same value. In case of the ring-assist-type MZ modulator 1020, such variations can be suppressed for the wavelength of the maximum modulation efficiency for the following reason. As illustrated in FIG. 1B, wavelength dependency of the amount of phase shift of the ring-assist-type MZ modulator 1020 is the sum of wavelength dependencies of amounts of phase shift of the individual rings 1023. A wavelength of the maximum modulation efficiency of the ring-assist-type MZ modulator 1020 exists in the neighborhood of an average of resonance wavelengths of the individual ring resonators 1023. Therefore, variations of the wavelength of the maximum modulation efficiency for the whole modulator are suppressed compared to variations of the resonance wavelengths of the ring resonators 1023.
On the other hand, the laser oscillator 1010 has only one ring resonator 1013, and hence, variations of the resonance wavelength of the ring resonator 1013 directly result in variations of the oscillation wavelength. Therefore, a shift is generated between the wavelength of laser oscillation (the graph in FIG. 1C) and the peak wavelength that maximizes the modulation efficiency (the graph in FIG. 1B). This shift reduces the modulation efficiency of the modulator 1020, or generates individual differences of the characteristic.
Note that a variable wavelength laser has been known that uses a loop filter configured with multi-stage waveguide ring resonators (see Patent Document 1, for example).